1. Field of the Invention
The present invention relates to a method for controlling current; in particular, to a method for controlling a three-phase current converter and device thereof.
2. Description of Related Art
Solar energy is a type of green energy that is virtually inexhaustible; therefore technology associated with the development of solar energy is currently in full swing. When a solar power device (i.e. solar panel) converts solar energy into electrical energy, this electrical energy can be directly incorporated into a city's local utility distribution network or stored within batteries. However, the lifespan of batteries are limited, and so the associated cost is relatively high. If a current converter is utilized, so that the solar energy is transformed into electrical energy, then the electrical energy can be directly incorporated into a city's local utility distribution network, thereby reduce the energy consumption during transmission and reduce power losses, resulting in a more efficient power generation system. In addition, the current converter can also be designed with two-way function, so that solar energy can selectively be supplied to DC loads directly, without the need to go through the distribution network into the city merely for the purpose of conversion into DC power, and this direct supply can minimize roughly 8% of power wastage. In regard to the options of two-way converter, system of 10 kW or more are primarily based on a three-phase design, as shown in FIGS. 1A and 1B, and can be categorized as either a delta connection three-phase current converter or a wye connection three-phase current converter, which better matches future power supply needs and system expansion. The control and reliability of three-phase current converter is an important issue for future research.
Traditional three-phase controlling method mainly utilizes space vector pulse width modulation (SVPWM) as the basis for developing current controller, based on reference, “M. P. Kazmierkowski and L. Malesani, Current Control Techniques for Three-Phase Voltage-Source PWM Converters: A Survey, IEEE trans, on Industrial Electronics, vol. 45, no. 5, pp. 691-703, October 1998”. However, according to references, “S. Fukuda and R. Imamura, Application of a Sinusoidal Internal Model to Current Control of Three-Phase Utility-Interface Converters, IEEE Trans. On Industrial Electronics, vol. 52, no. 2, pp. 420-426, March. 2005” and “Q. Zeng and L. Chang, An Advanced SVPWM-Based Predictive Current Control for three-Phase Inverters in Distribution Generation Systems, IEEE Trans. On Industrial Electronics, vol. 55, no. 3, pp. 1235-1246, March. 2008” SVPWM is based on the condition of balanced three-phase voltage, and from that basis current error compensation is utilized to overcome issues such as harmonic distortion from a city's electric utility or control problems from sampling delays. Furthermore, according to reference, “K. M. Smedley and C. Qiao, Unified constant-frequency integration control of three-phase power factor corrected rectifiers, active power filters and grid-connected inverters, U.S. Pat. No. 6,545,887, Apr. 8, 2003”, K. M. Smedley proposed dual-buck control method to simplify complicated derivation resulting from traditional SVPWM however such derivation is still based on condition of a balanced three-phase inductance.
It should be noted; three-phase inductance value is not a constant value that is unchanging, see FIG. 2 which shows a diagram of inductance variation vs. current for a magnetic moly-permalloy powder core (MPP core) winding of a 10 kW three-phase system. As shown by FIG. 2, the greater the power within a system, the current is correspondingly greater, and so the inductance is correspondingly smaller. Thereby since a SVPWM is based on condition of a balanced three-phase inductance for directing and deriving controlling method, so when inductance changes due to variance in current, the directing and deriving controlling method for a SVPWM would fail to establish.
If a controller does not take into account of the variance of inductance, then massive compensation must be made to overcome the lack of inductance value, so that such a system would be at risk of divergence. Therefore, a controlling method that accounts for inductance variation is necessary. Additionally, for a general current converter controller, feedback sampling is often interfered by noises generated from switching of switches, resulting in controller oscillating or false action. Although one can use an analog filter to remove high frequency noise, such solution would result in delay of feedback signal and subsequently lead to a slower system response, so that the AC output of the current converter becomes distorted. Thereby in recent years relevant industry begins utilizing digital signal processor (DSP) for pulse width modulation control, so as to retrieve multiple feedback signal samples within one switching cycle for averaging, so as to reduce the effect of high frequency noise. However, multiple sampling does not actually match feedback current value to reference current value, but rather increases DSP processing and calculation time.